Glass optical articles, such as aspheric lenses, ball-shaped lenses, prisms, etc are generally made by a direct press-molding process using a mold. The glass optical articles obtained by the direct press-molding method advantageously do not need to undergo further processing, such as a polishing process. Accordingly, the manufacture efficiency can be greatly increased. However, the mold used in the direct press-molding method has to satisfy certain critical requirements such as high chemical stability, resistance to heat shock, good mechanical strength, and good surface smoothness.
Several criteria that should be considered in choosing the material for making the mold are listed below:                a. the mold formed from such material is rigid and hard enough so that the mold cannot be damaged by scratching and can withstand high temperatures;        b. the mold formed from such material is highly resistant to deformation or cracking even after repeated heat shock;        c. the mold formed from such material does not react with or adhere to the glass material at high temperatures;        d. the material is highly resistant to oxidization at high temperatures;        e. the mold formed of such material has good machinability, high precision, and a smooth molding surface; and        f. the manufacturing process using the mold is cost-effective.        
In earlier years, the mold was usually made of stainless steel or a heat resistant metallic alloy. However, such mold typically has the following defects. Sizes of crystal grains of the mold material gradually become larger and larger over a period of time of usage, whereby the surface of the mold becomes more and more rough. In addition, the mold material is prone to being oxidized at high temperatures. Furthermore, the glass material tends to adhere to the molding surface of the mold.
Therefore, non-metallic materials and super hard metallic alloys have been developed for making molds. Such materials and alloys include silicon carbide (SiC), silicon nitride (Si3N4), titanium carbide (TiC), tungsten carbide (WC), and a tungsten carbide-cobalt (WC—Co) metallic alloy. However, SiC, Si3N4 and TiC are ultrahard ceramic materials. It is difficult to form such materials into a desired shape, especially an aspheric shape, with high precision. Further, WC and a WC—Co alloy are liable to be oxidized at high temperatures. All in all, these materials are not suitable for making high-precision molds.
Thus, a composite mold comprising a mold base and a protective film formed thereon has been developed. The mold base is generally made of a carbide material or a hard metallic alloy. The protective film is usually formed on a molding surface of the mold base.
Typically, the mold base of the composite mold is made of a hard metallic alloy, a carbide ceramic, or a metallic ceramic. The protective film of the composite mold is formed of a material selected from the group consisting of iridium (Ir), ruthenium (Ru), an alloy of Ir, platinum (Pt), rhenium (Re), osmium (Os), rhodium (Rh), and an alloy of Ru, Pt, Re, Os and Rh.
U.S. Pat. No. 5,202,156 discloses a method for making a composite mold. In this method, a mold base of the mold is made of a hard metallic alloy, a carbide ceramic, or a metallic ceramic. A diamond like carbon film is formed on a molding surface of the mold base as a protective film.
However, after a period of usage, the protective film is apt to crack or even peel off. Therefore, the quality of the glass products made by such mold is decreased, and the service lifetime of such mold may be shortened.
In view of the above shortcomings, a middle layer has been devised. The middle layer is arranged between the mold base and a noble metal protective film formed on the mold base. The material of the middle layer should have characteristics such as good machinability, high chemical stability, a high melting point, and resistance to migration of atoms. The material of the mold base should have characteristics of good heat resistance, high hardness, and excellent machinability. The noble metal protective film should have anti-oxidation characteristics and good smoothness. In general, the mold base is made by sintering a carbide material or a metallic ceramic. In the sintering process, metal elements such as cobalt (Co), nickel (Ni) or molybdenum (Mo) are generally introduced as additives. When the mold is in use, heat gradually accumulates therein and generally cannot be dissipated efficiently. Therefore, the temperature of the mold base is unduly elevated. After a period of repeated usage, the protective film is liable to peel off from the mold base. Therefore, the service lifetime of the mold may be shortened. In addition, the metal elements may migrate to a surface of the mold and react with the glass material. Therefore, the quality of the glass products formed may be diminished.
Furthermore, when the mold is in use, the accumulated heat may reach a level at which the mold must be temporarily decommissioned and allowed to cool. During this period of downtime, production must be halted or a backup mold must be commissioned. Therefore, the efficiency of mass manufacturing is decreased. Moreover, the mold is usually cooled by a gas cooling method. Commonly, nitrogen (N2) gas is used as a cooling gas for cooling the mold. However, a large amount of N2 gas is consumed in order to achieve satisfactory cooling.
Therefore, a low cost composite mold with good heat dissipation ability for improved manufacturing efficiency is desired.